If the physics engine is the stage, the animated characters are the puppets. Ragdoll physics look realistic because these characters are made up of rigid parts connected to one another in a system that's similar to real-world skeletal bodies. When damaged, the bodies flop, loll and bounce around onscreen.
The math and physics at play are exceedingly complex, and even now CPU power and processing algorithms haven't quite found a way to perfectly mimic a collapsing humanoid form. Thus, hilarity often ensues as the articulated limbs of the character twist and bounce in all sorts of unrealistic and absurd ways, like a ragdoll flung down a flight of stairs. Plenty of current online games, such as "Happy Wheels" capitalize on this effect, essentially providing crash-test dummies for you to torture and fling through all sorts of silly and bizarre scenarios.
Many simple contemporary games leverage ragdoll physics as a way to make onscreen action less violent and more cartoonish and fun. Yet the amusement and entertainment factors masks the complexity of the physics that go into contemporary games.
One of the most sophisticated animation engines is called Euphoria, made by NaturalMotion. Euphoria has been used in some exceedingly popular games, such as "Star Wars: The Force Unleashed," "Red Dead Redemption" and "Grand Theft Auto 4."
NaturalMotion boasts that Euphoria simulates bodily motion in real time for each and every part of every scene, right down a character's muscles and motor system. For instance, characters might be startled by the sound of a machine gun blast or try to break their fall when they topple from a moving vehicle. In the end, these kinds of games offer a more absorbing gaming experience.